The present invention relates to a device for ultrasonic treatment of workpieces.
Devices of this type are used for connection of two components by welding or riveting with the use of ultrasound. They also can be used for other purposes, for example for cutting and the like. The treatment is performed so that an electrode which is identified as a sonotrode is subjected to mechanical vibrations by a converter which converts electrical energy into mechanical energy, and is pressed in the region of a preselected treatment point against the workpiece so as to plasticize the workpiece over a short time by heating. Such methods are utilized nowadays in many areas, for example in automobile industry and used there in particular for production of shock absorbers, door and trunk space coatings, instrument panels, consoles and the like of polypropylene and other thermoplastic materials.
In the known devices of this type, for example as disclosed in the German patent document DE 42 06 584 C2, each resonant unit (or vibration structure) which is composed of a converter, a sonotrode and in some cases a booster (or amplitude transformer) cooperating within them, are mounted on an associated displacement unit. The displacement unit is composed of a controllable, hydraulic or pneumatic cylinder/piston unit and displaces the resonant unit in direction of the workpiece to be treated. The sonotrode, depending on the position and construction of the treatment spots, can be composed of a one-piece vibration body or of a carrier body with at least one daughter horn (screw-in pin). The vibration body or the daughter horn is provided as a rule with a working or welding surface extending perpendicular to the direction of displacement. Thereby, for example in the case of a welding or riveting, it is guaranteed that the welding or riveting dome which is formed on the workpiece and applied on one of the two components to be connected can extend through corresponding openings of the other component. It can be for example 10 mm high and 1-3 mm wide, it can be loaded exactly in its axial direction, and therefore with conventionally utilized pressing forces of for example 200 N during the softening it does not move back or curve laterally.
The German reference DE 44 39 470 C1 discloses further arrangements of the above mentioned type in which the sonotrodes are connected with at least two converters. This guarantees that the sonotrodes with large treatnent surfaces are driven uniformly into vibrations over their full expansion.
During the utilization of known arrangements of the above mentioned types, the existing problems are, not resolved satisfactorily always when the workpiece has a plurality of treatment spots, for example in form of welding or riveting spots, which are located at different heights, in particular when they are located closely near one another and the distances between then are for example less than 50 mm. Since the whole resonant unit must be determined always within narrow limits of a preselected resonance frequency of for example 35 kHz, the sonotrode must have throughout substantially the same height corresponding to an integral multiplicity of the half wave length. With the use of several daughter horns per sonotrode, the pins must also have substantially the same length. This practically excludes the simultaneous treatment of treatment spots located at different height levels with the same sonotrode, since the daughter horns or sonotrode surfaces which deviate from one another in direction of the height by only several millimeters or have other asymmetricities can lead to fast distortion of a corresponding converter.
Known optimization methods for the sonotrodes, such as disclosed for example in the German patent document DE 31 43 461 A1, help this problem only conditionally. As a result, a special resonant unit which is adapted for each treatment welding location is needed, or the closely adjacent treatment pots must be treated frequently in several, successive cycles, since they are not simultaneously accessible with the available sonotrodes, in particular when the treatment direction must extend parallel to the axial direction of the riveting dome or the like. This increases the treatment times and requires a high expense for the holding of a plurality of different sonotrodes.